The invention relates to data modems and more particularily to expandable data modems of modular construction.
Data modems are widely used for transmitting digitally encoded information through information channels, that are typically of bandwidth limited type, such as voice channels that have a bandwidth substantially in the range from 250-3500 Herz.
Digital information, before it can be transmitted through such a channel, normally must be converted to a form that is suitable for transmission over long distance-voice channels.
Long distance voice channels typically include many transmission improving and correcting devices, such as amplifiers, inductive loading coils and so forth, which enhance voice transmission but in turn in certain respects present problems for data transmission.
In order to be transmitted over such voice channels, the data must first be modulated to a suitable form before they enter the line. When the modulated data enter the other end of the voice channel, they must again be demodulated back to their original form. In a two-way data channel, there is therefore a combined modulator-demodulator at each end, usually called modem.
Modems for low data speeds, such as e.g. 300-1200 data bits per second also called "bauds per second" or simply "baud" are typically quite simple devices, consisting of a combined frequency-shift modulated oscillator combined with a frequency-modulated detector at each end. Such simple modems, however, are usually suitable only for limited data speeds. When greater data speeds are required, modems of more complex construction are required, which use complex modulation techniques for overcoming the physical limitations of the voice and data channel. It follows that these more complex modems typically require auxilliary circuits to support them; they are also more complex and accordingly more expensive, and the more so, the higher the data rate.
Such auxiliary circuits include the power supply, the clock generator and line equalizing circuitry for improving certain transmission parameters of the voice channel. It follows that all these auxiliary circuits as well as the modem become more complex at high data rates.
A modem is typically inserted between a computer and a data line, and as such includes a number of circuits that, to one side, interact with the data line for controlling it, and to the other side, interact with the computer. These are basic circuits that are required, almost identically the same for every modem.
With the present state of the art, when one buys a modem for a computer, it may either be a standalone modem that is logged in via a RS232 or similar port on the computer, or it may be a drop-in card for the personal computer. There are a number of different modems of this particular fashion, many of them are for 300 baud, and many of them are either 300 or 1200 baud but selectable. Some of the more expensive modems go up to 300/1200/2400/4800, baud or higher. In the past, because of the slowness of technology, one would create a modem to handle data at a certain baud rate or a certain range of baud rates, but as the state-of-the-art advances and the need arises for a better modem, it is always necessary to buy a completely new modem and to discard the old one, even if possibly fifty percent of the components on the old modem would be wasted, because they would again be required for the new modem.
It is therefore a primary object of the present invention to provide a type of modem that is upgradable to higher and better performance without the need for completely discarding the entire modem, but that is constructed such that only a small part of the modem can be taken out and replaced with a new part, so that the upgrading can be done at much less expense, than required if the entire modem is replaced.